Process for heat-treating polyamide filaments



United States Patent 3,546,329 PROCESS FOR HEAT-TREATING POLYAMIDEFILAMENTS Koyu Hirono and Ken Kuwano, Mihara-shi, Japan, assignors toTeijin Limited, Umeda, Osaka, Japan, a corporation of Japan No Drawing.Filed Dec. 18, 1967, Ser. No. 691,167 Claims priority, applicationJapan, Dec. 16, 1966, 41/ 82,468 Int. Cl. B29c 25/00 US. Cl. 264--235 5Claims ABSTRACT OF THE DISCLOSURE A drawn polyamide filament issubjected to one or more tensions applied in positive increments at afilament temperature of between about 180 C. and about 210 C. so thatthe initial tension on the filament lies within the range of betweenabout 0.20 gram/ denier and about 2.0 grams/denier while the finaltension on the filament lies within the range of about 0.50 gram/denierand about 2.0 grams/denier. The filament is heated by means of an inertatmosphere and each tension is applied to the filament for a period oftime of between about 1 second and about seconds. Filaments so treatedhave enhanced dimensional stability, heat resistance, moistureresistance, and fatigue resistance as well as undiminished tensilestrength.

This invention relates to the heat treatment of polyamide filaments.More particularly, it relates to the application of heat and tension tonylon-6 filaments to improve the dimensional stability, heat resistance,moisture resistance, and fatigue resistance thereof.

The dimensional stability of presently available polyamide fiber underconditions of stress, particularly at elevated temperatures is generallyinsufficient to permit optimum use of such fiber in all thoseapplications to which it is potentially suited. Lack of dimensionalstability under high stress is manifested, for example, in thephenomenon of fiat-spotting generally exhibited by automobile tirescontainning cord made from polyamide filaments which tend to elongateeven under normal tire load conditions.

Previous attempts to treat polyamide filaments after drawing to enhancethe dimensional stability thereof have been only partially successful;the dimensional stability cannot be significantly enhanced without atthe same time sacrificing some of the tensile strength. It is alsodesirable to improve the properties of fatigue resistance, moistureresistance and heat resistance simultaneously with the improvement indimensional stability without concomitant loss of tensile strength,which tensile strength is preferably greater than about 9.09.5grams/denier.

Therefore, it is an object of the present invention to provide a processfor improving the dimensional stability, fatigue resistance, moistureresistance, and heat resistance of a polyamide fiber without impairingthe tensile strength and uniformity thereof.

This and other objects as well as a fuller understanding of the presentinvention can be had by reference to the following description andclaims.

According to the present invention, a spun polyamide filament which hasbeen drawn to the desired denier and tensile strength is exposed to acombination of judiciously ice selected conditions of tension,temperature, and atmosphere for a period of time so as to impart to thefilament improved dimensional stability, heat resistance, moistureresistance, fatigue resistance and undiminished tensile strength.

More particularly, according to the present process, a spun polyamidefilament, preferably a filament of nylon-6, which has been drawn to thedesired denier and tensile strength is heated to a temperature ofbetween about C. and about 210 C., and preferably between about C. andabout 210 C. Simultaneously, a series of one or more tensions is appliedto the filament, beginning with a tension of between about 0.20gram/denier and about 0.80 gram/denier and ending with a tension ofbetween about 0.5 gram/denier and about 2.0 grams/ denier. Where theprocess is adapted to consist essentially of a single application oftension, such tension is preferably between about 0.5 gram/denier andabout 2.0 gram/denier.

It is an essential feature of .the invention that, except for theinitially applied tension, each tension which is applied to the filamentmust be greater than the preceding tension, i.e., all increments intension must be positive. The duration of application of a given tensionranges from about 1 second to about 10 seconds, and preferably betweenabout 2 seconds and about 6 seconds.

The present process is applicable to the heat treatment of polyamidefiber in any form. Thus, the process can be applied to theheat-treatment of an individual filament or to a plurality thereof,either as Zero twist yarn or as yarn which has a twist imparted thereto.

It is a preferred feature of the present invention that the heattreatment of the polyamide filament is performed in such a way that thefilament need not come into contact with any solid heating elementsduring such treatment. The use of solid heating elements to heat thefilament via contact therewith can impair the quality and uniformity ofthe filament, mainly because of the difficulty in securing accuratecontrol of such heating elements at the high temperatures employed inthe present process. In the present invention on the other hand, it ispreferred that the filament be heated by means of a fluid heat transfermedium which is chemically unreactive toward the filament at thetemperatures employed. Those solid objects, e.g., tensioning rollers,which do contact the filament during the heat treatment for the purposeof applying tension thereto are not required to be at a highertemperature than the fluid transfer medium and are preferably maintainedin thermal equilibrium with the filament. Consequently, the undersirableeffects of contact between the filament and the solid object areminimized.

A fluid heat transfer medium suitable for use in the present process maybe liquid or gaseous. Preferably, the fluid heat transfer medium is agas, e.g., air, superheated steam, nitrogen, carbon dioxide, and thelike, or mixtures thereof. In further connection with the heatingatmosphere of the present process, the temperature of such an atmosphererequired to sustain a filament temperature of between about 180 C. andabout 210 C. is between about 200 C. and about 300 C. The filamenttemperature is preferably determined by non-contact methods conventionalto the thermometric arts, e.g., by means of a non-contaet-type runningfilament thermometer.

It is necessary that the temperature of the filament in each stage ofthe heat treatment process be maintained between about 180 C. and about210 C. Filament temperatures below about 180 C. are ineffectual;filament temperatures above about 210 C. cause deterioration of thefilament with accompanying loss of tensile strength. The filamenttemperature of each stage is at least as high as, and preferably equalto the filament temperature of the preceding stage. Furthermore, it isnecessary to conduct each stage of application of tension(tensilization) for a period of no less than about 1 second and no morethan about seconds. Tensilization for less than about 1 second isineffectual. Tensilization for more than about 10 seconds results indecreased tensile strength due to thermal deterioration.

According to the present invention, the number of stages of successivelyincreasing tension can be one or any number greater than one, providedthat each tension applied to the filament is greater than thatpreceding. When the present process is conducted using a singleapplication of tension, it is desirable that such tension have a valueof between about 0.50 gram/denier and about 2.0 grams/ denier. If thetension is less than about 0.5 gram/denier, the tensile strength andYoungs modulus are undesirably diminished while the denier of thefilament is undesirably increased. If the tension is greater than about2.0 grams/ denier, the tensile strength is diminished and the heatresistance of the filament is not substantially improved. In aparticularly preferred mode of conducting the present process using asingle application of tension, such tension has a value of between about0.5 gram/denier and about 0.8 gram/denier.

When a two-stage process is employed, it is preferred that the initialtension have a value between about 0.20

gram/denier and about 0.80 gram/denier; the tension in the final stageis preferably between about 0.50 gram/ denier and about 2.0grams/denier. If the tension applied in the first stage is less thanabout 0.20 gram/denier, the tensile strength and Youngs modulus areundesirably lowered and this result cannot be remedied in the secondstage regardless of the'tension applied therein. If the tension appliedin the second (and final) stage is less than about 0.5 gram/denier, thetensile strength and Youngs modulus are undesirably diminished while thedenier of the filament is undesirably increased, since the tensionapplied in the preceding stage must also be less than about 0.5 gram/denier. If the tension applied in the second stage is greater than about2.0 grams/denier, the tensile strength of the filament is lowered andthe heat resistance thereof is not substantially improved. In aparticularly preferred two-stage process, a tension of between about0.20 gram/ denier and about 0.30 gram/denier is employed in the firststage, and a tension of between about 0.50 gram/ denier and about 1.0gram/denier is employed in the second stage. In the first stage of thetwo-stage heat treatment, the dimensional stability of the filament isimproved; the second stage is effective in maintaining high tensilestrength.

When a three-stage process is employed, it is preferred that the initialtension have a value of between about 0.50 gram/denier and about 0.80gram/denier. The tension in the second stage is preferably between about0.8 gram/ denier and about 1.3 grams/denier. The tension in the thirdstage is preferably between about 1.3 grams/denier and about 1.8grams/denier.

In general, the aforementioned Objects of the present invention withregard to dimensional stability, fatigue resistance, heat resistance,moisture resistance, and tensile strength are most satisfactorilyachieved by operating at higher temperatures, longer heating times, andlower tensions consistent with the limits of those conditions set forthhereinabove.

Polyarnide filament heat treated according to the process of the presentinvention have increased density and melt temperature. These, togetherwith analytical measurements to be described hereinafter in connectionwith Examples I-III, indicate that polyamide filaments so treated havean enhanced crystalline structure compared with untreated filaments.Without wishing to be bound by theory, it is suggested that theimprovement in dimensional stability, heat resistance, moistureresistance and fatigue resistance are consequences of the improvedcrystalline structure of the polyamide filaments.

The process of the present invention is preferably applied to the heattreatment of Nylon-6 fiber. Spun and drawn filaments of Nylon-6 incondition for heat treatment according to the present process can beprepared, for'example, by spinning a polycaproamide of formic acidrelative viscosity preferably between about and about and having a melttemperature of preferably between about 270 C. and about 290 C. Afterbeing cooled, oiled and reeled, the filaments of Nylon-6 are drawn at adraw ratio of preferably between about 4.0 and about 6.0.

The process of the present invention can be conducted using anyapparatus which is suitably adapted to effectuate the heat treatment ofpolyamide fiber in the manner disclosed hereinabove. By way of example,a suitable sys tem of conventional draw rollers can be advantageouslyused in conjunction with conventional fluid heating means to secure thesequential tensioning at the appropriate temperatures and for theprescribed durations of tensioning in accordance with the presentinvention.

It is a feature of the present process that there is provided thereby aNylon-6 fiber of high tensile strength (i.e., in excess of 9.09.5grams/denier), and improved dimensional stability under conditions ofstress and elevated temperature (manifested by the facts that shrinkageof the yarn in boiling water is decreased by 48% and tensilized thermalshrinkage, as described in Example I, is decreased by 13% Fatigueresistance is generally increased by about 20-70% and moisture and heatresistance are generally increased by about 5-15%.

The following examples illustrate the application of the process of thepresent invention to the heat-treatment of a 204-filament Nylon-6 yarnhaving a total denier of 1200 and a twist number of 39 x 39 twists/10cm. (ASTM). The yarn is prepared by spinning and drawing Nylon-6 havinga formic acid relative viscosity of 90.

Tensile strengths are determined by means of a conventional Instron-typetension meter.

Filament temperatures are determined by means of a non-contact runningfilament thermometer.

EXAMPLE I The 1200 denier/204-filament yarn is subjected to asingle-stage heat treatment in an atmosphere of air maintained at atemperature of 240 C. The yarn is exposed to the heated air for 6seconds while under a tension of 0.70 gram/ denier. The yarn reaches amaximum temperature of 210 C., which temperature subsists for 3 seconds.A comparison of some of the physical properties of the heat treated yarnwith the corresponding properties of the same yarn prior to treatment ispresented in Table I.

TABLE I.SINGLE-STAGE HEAT TREATMENT n This term refers to the degree ofshrinkage, after 30 minutes in air at C., of yarn having an initiallength of 50 centimeters and an initial tensile load thereon of 5 20,gram/denier.

b This property is measured by the tube fatigue testing method of J ISLI017 1963. Values shown are ratios of the fatigue resistance of testedyarn to that of untested yarn.

This property is expressed in terms of space indiees as determined byOdaka et. al., in Chem. High. Polymers (Japan), 7, 122 (1950).

d This property is a measure of the average repeat period of crystallineregions and amorphous regions. Long period is measured by conventionalX-ray small angle scattering techniques.

EXAMPLE II The 1200 denier/204-filament yarn is subjected to a two-stageheat treatment in an atmosphere of nitrogen maintained at a temperatureof 270 C. in both stages. The first stage of the process is conductedfor 4 seconds during which time a tension of 0.23 gram/denier is appliedto the yarn; the maximum temperature of the yarn in this stage is 210C., which temperature subsists for 2 seconds. The second stage of theprocess is conducted for 4 seconds during which time a tension of 0.52gram/ denier is applied to the yarn; the maximum temperature of the yarnin this stage is 210 C., which temperature subsists for 2 seconds. Acomparison of some of the phys ical properties of the heat treated yarnwith the corresponding properties of the same yarn prior to treatment ispresented in Table II, wherein the tabulations have the same meaning asin Table I.

TABLE II.TWO-STAGE HEAT TREATMENT EXAMPLE HI The 1200denier/204-filament yarn is subjected to a three-stage heat treatment inan atmosphere of air maintained at 250 C. in the first stage, 260 C. inthe second stage, and 270 C. in the third stage. The residence time ofthe yarn in each stage is 4 seconds. In the first stage, a tension of0.8 gram/denier is applied to the yarn; the maximum temperature of theyarn in the first stage is 195 C., which temperature subsists for 1.4seconds. In the second stage, a tension of 1.2 grams/ denier is appliedto the yarn; the maximum temperature of the yarn in the second stage is203 C., which temperature persists for 1.7 seconds. In the third stage,a tension of 1.6 grams/ denier is applied to the yarn; the maximumtemperature of the yarn in the third stage is 210 C., which temperaturepersists for 2.0 seconds. A comparison of the quality of the yarnsubjected to this three-stage heat-treatment is compared with thequality of the same yarn in an untreated state. The results of thiscomparison are summarized in Table III wherein the tabulations have thesame meaning as in Table I.

TABLE III.THREE-STAGE HEAT TREATMENT Yam treated Untreated according toyarn Example III Yarn strength (grams/denier) 9. 5 9. 5 Yarn shrinkagein boiling water (percent) 11. 0 6. 0 Young's modulus (kg/mm. 400 450Tensilized thermal shrinkage (percent)- 13. 0 11. Fatigue resistanceOlgg 012g Moisture and heat resistance R a This term refers to thedecrease in tensile strength of yarn having an initial length of 30centimeters and an initial load thereon of /2a gram per denier aftersubjecting the yarn to a saturated water vapor pressure of 5.5 kg./cm.at 150 C. in an autoclave for mm tes. Values shown are atios of thetensile strength of tested yarn to that of untested yarn.

comprising:

(a) heating the filament at a temperature of between about C. and about210 C.;

(b) applying a tension to the filament of between about 0.20 gram/denier and about 0.80 gram/denier.

(c) conducting step (b) for a period of time of between about 1 secondand about 10 seconds;

(d) applying a positive increment of tension to the filament obtained instep (c) at the temperature of step (a) or higher but below 210 C. togive a resulting tension on the filament of between about 0.5gram/denier and about 2.0 grams/denier; and

(e) applying each resulting tension in step (d) for a period of time ofbetween about 1 second and about 10 seconds.

2. A process according to claim 1 wherein the polyamide filament is afilament of polycaproamide;

the heating step (a) is conducted at a temperature of between about C.and about 210 C. in an inert, fluid heat-transferring medium;

the tensioning step (b) is conducted at a tension of between about 0.2gram/denier and about 0.3 gram/ denier;

step (b) is conducted for a period of time of between about 2 secondsand about 10 seconds;

the increment of tension applied in step (d) is between about 0.20gram/denier and about 0.8 gram/ denier to give a resulting tension onthe filament of between about 0.5 gram/denier and about 1.0 gram/denier; and

the resulting tension in step (d) is applied for a period of time ofbetween about 2 seconds and about 10 seconds.

3. A process according to claim 2 wherein the polycaproamide ischaracterized by having a formic acid relative viscosity of betweenabout 60 and about 140 and a melt-temperature of between about 270 C.and about 290 C.;

the polycaproamide filament, prior to treatment, is characterized inhaving been drawn to a draw ratio of between about 4.0 and about 6.0 andhaving a tensile strength of at least about 9.0 grams/ denier; and

the fluid heat-transferring medium is selected from the group consistingof air, super-heated steam, nitrogen, and carbon dioxide.

4. A process according to claim 1 wherein the polyamide filament is afilament of polycaproamide;

the heating step (a) is conducted at a temperature of between about 195C, and about 210 C. in an inert, fluid heat-transferring medium;

the tension applied to the filament in step (b) is between about 0.5gram/ denier and about 0.8 gram/ denier;

step (b) is conducted for a period of time of between about 2 secondsand about 10 seconds;

step ((1) comprises:

(A) applying a positive increment of tension to the filament obtained instep (c) to give a tension on the filament of between about 0.8 gram/denier and about 1.3 grams/ denier; and

(*B) applying a positive increment of tension to the filament obtainedin step (A) to give a tension on the filament of between about 1.3grams/ denier and about 1.8 grams/denier; and

each tension in step (d) is applied for a period of time of betweenabout 2 seconds and about 6 seconds.

5. A process according to claim 4 wherein:

the polycaproamide is characterized by having a formic acid relativeviscosity of between about 60 and about 140 and a melt-temperature ofbetween about 270 C. and about 290 C.;

the polycaproamide filament, prior to treatment, is characterized inhaving been drawn to a draw ratio of between about 4.0 and about 6.0 andhaving a tensile strength of at least about 9.0 grams/ denier; and

the fluid heat-transferring medium is selected from the group consistingof air, superheated steam, nitrogen, and carbon dioxide.

References Cited UNITED STATES PATENTS Lewis 264342UX Miles 264342UXSchenker 264342UX T anabe et a1. 264346X McColm et a1. 264346X 10 8FOREIGN PATENTS 12/1966 Japan 264346 JULIUS FROME, Primary Examiner J.H. WOO, Assistant Examiner US. 01. IX.R.

